Large volumes of natural gas (i.e., primarily methane) are located in remote areas of the world. This gas has significant value if it can be economically transported to market. Where the gas reserves are located in reasonable proximity to a market and the terrain between the two locations permits, the gas is typically produced and then transported to market through submerged and/or land-based pipelines. However, when gas is produced in locations where laying a pipeline is infeasible or economically prohibitive, other techniques must be used for getting this gas to market.
A commonly used technique for non-pipeline transport of gas involves liquefying the gas at or near the production site and then transporting the liquefied natural gas to market in specially designed storage tanks aboard transport vessels. The natural gas is cooled and condensed to a liquid state to produce liquefied natural gas (“LNG”). LNG is typically, but not always, transported at substantially atmospheric pressure and at temperatures of about −162° C. (−260° F.), thereby significantly increasing the amount of gas which can be stored in a particular storage tank on a transport vessel. For example, LNG takes about 1/600 of the volume of natural gas in the gas phase.
Once an LNG transport vessel reaches its destination, the LNG is typically off-loaded into other storage tanks from which the LNG can then be revaporized as needed and transported as a gas to end users through pipelines or the like. LNG has been an increasingly popular transportation method to supply major energy-consuming nations with natural gas.
The liquefaction process may have a number of stages during which the natural gas is cooled and liquefied. During the cooling process, the pressure is lowered, with the shipping pressure of the liquefied product being near atmospheric (for example, about 3.6 psig or less). The decrease in pressure assists in cooling the natural gas during the liquefaction process by decreasing the enthalpy of the natural gas. Refrigeration equipment is also used for removing heat energy. One stage of this process requires that the high-pressure natural gas stream be reduced in pressure sufficiently to assist in the production of extremely cold LNG (or subcooled LNG) by extracting energy (or enthalpy) from a liquid natural gas stream.
Pressure drop in a hydraulic turbine can often be used in LNG processes to remove energy from refrigerant streams and natural gas streams, or other systems, to obtain lower temperatures. The energy removed from these streams may also be used to generate electrical power. For example, turbines can be coupled with a generator to provide the braking load necessary to remove the energy. The generator may be coupled to the facility power grid, wherein the additional power improves the thermodynamic efficiency of the process. In LNG processes, the efficiency improvement may be about 1 to 2%, resulting in saving many Megawatt-hours per year and improving economic justification of the liquefaction process.
Other parties have proposed the concept of applying turbines in series to satisfy the need for high pressure let down at a magnitude greater than typically performed in existing facilities. Examples of series expansion are considered in patents related to air separation, as well as in cascade LNG liquefaction processes, among others.
U.S. Pat. No. 3,724,226 to Pachaly discloses an LNG expander cycle process employing integrated cryogenic purification. In the process, a work-expanded refrigerant portion undergoes a compression cycle and is work expanded through a series of expansion turbines. The expansion turbines furnish at least part of the power necessary to drive the compressor system in the refrigerant gas cycle, by sharing a common shaft or other mechanical coupling with the compressors. The expanders used are turbo-expanders, which can liquefy a portion of a high-pressure gas stream as it is depressurized through the turbo expanders. The expanded stream can then be flowed through cooling units to remove more energy, prior to flowing through more turbo-expanders.
U.S. Pat. No. 4,019,343 to Roberts discloses a refrigeration system using enthalpy converting liquid turbines. The refrigeration system uses a series of liquid turbines, each of which have an associated compressor. A stream of liquid ammonia is allowed to expand in a liquid turbine, during which a portion of the liquid flashes and is sent to the associated compressor. The cooled, expanded liquid flows to the next turbine in the series, where the process is repeated.
Related information may be found in U.S. Pat. Nos. 2,922,285; 3,677,019; 4,638,638; 4,758,257; 5,651,269; 6,105,389; 6,647,744; 6,898,949; and 7,047,764. Further information may also be found in United States Patent Application Publication Nos. 2003/0005698 and 2005/0183452. Additional information may be found in International Patent Application Publication No. WO 2007/021351 and European Patent Application Publication No. 0 672 877 A1.
Current turbine expanders are centrifugal expanders that have a series of blades forming a triangular shape over a central shaft forming an impeller. A high pressure stream enters from a port over the tip of the blades at the outer diameter of the impeller, flowing radially inward, rotates the impeller, turning the shaft, and a lower pressure stream exits through an outlet from the center of the shaft flowing out axially from the impeller. Each expander generally has a single expander impeller. Thus, these turbine expanders have a relative small flow rate and pressure drop. Accordingly, there is a need for a plurality of the centrifugal expanders when generating liquefied gas in a large facility. For this reason, centrifugal expanders are not preferred for use in large plants due to increased costs, increased space requirements, and maintenance characteristics that are complicated by the increased amount of machinery. The efficiency of centrifugal expanders is generally also limited to about the low eighty percent level. Furthermore, an axial expander can provide higher efficiency resulting in more liquids or sub-cooling of the fluid for the same pressure drop which can be very valuable in gas liquefaction and gas separation processes. Further, axial turbines have been developed extensively as steam turbines but have very limited application for gas expanders.
For example, Japanese Patent Publication No. 2003-27901 by Ono et al. (hereinafter “the Ono patent”), discloses an axial flow expander. The axial flow expander is provided with a turbine stage configured from a stationary blade affixed to a stationary body and a moving blade affixed to a turbine rotor. A working fluid flow path, having a plurality of turbine stages, is aligned in the axial direction of the turbine. A bypass flow path is provided outside the working fluid flow path to allow part of the working fluid flowing in from the upstream side in the flow direction of the working fluid to bypass the outer peripheral side of at least one of the turbine stages and to be introduced into a turbine stage located on the downstream side in the flow direction of the working fluid from the bypassed turbine stage.
However, it is often necessary for the rotary machine to be available to remove the inner components from a casing for servicing. The axial flow expander disclosed in the Ono patent does not provide such a structure for removing the inner components from the casing. In fact, the fixed structures inside the flow path prevent removal without a structure for opening the case. To allow for assembly and servicing the Ono patent allows the casing to be divided into two parts in a horizontal plane. However, the dividing structure of the case may lower the pressure that can be retained by the turbine expander. In an expander used in a refrigeration cycle, or for process gas cooling the inside of the casing is subject to high-pressure. For this reason, there may be a leak of the gas from the division surface.
Accordingly, there is a need for an axial flow expander that is capable of increasing flow rates, increased pressure drop, higher efficiency and sealing in high pressure gases.